Containers are currently being produced with exceedingly thin walls, having a thickness of 0.0038 inches or even less, which are obtained by first forming a flat circular metal disk into a shallow cup, reforming the shallow cup to produce a wall of intermediate length and then ironing the wall to reduce the thickness thereof and increase the height of the container shell. In this type of process, the integral or unitary end wall of the container is usually approximately the same thickness as the stock material that is utilized for forming the cup. Usually the end wall is reformed to some configuration, normally an inwardly domed configuration to increase the strength thereof.
The container shells formed of the above type are normally produced by what is commonly referred to as a drawing and ironing press which has a ram or punch that is reciprocable along a path and has a series of ironing dies aligned therewith as well as a redraw die at one end of the ironing dies and a stripper assembly at the opposite end. In most commercial presses, the press or drawing and ironing machine also includes a domer assembly that is positioned at the end of the stroke of the ram to cooperate with the ram or punch and reform the end wall at the end of the drawing and ironing operation.
One type of machine that has been utilized for producing container shells of the above type is manufactured by Ragsdale, Inc. and is identified as a model CR-24 Can Wall Drawing and Ironing Press. This machine includes a plurality of axially spaced die assemblies that cooperate with the movable punch to convert a shell cup into a finished container shell. At the end of the stroke, the punch cooperates with the domer assembly for producing the final configuration of the integral end wall of the container. Usually such end wall configuration is domed inwardly, as for example, is shown in the U.S. Pat. No. 3,942,673.
To produce such configuration, various types of doming assemblies have been proposed for cooperating with a punch to reform an end wall of a container shell. One example of such doming assemblies is disclosed in U.S. Pat. No. 3,491,574 wherein a fixed domer element is positioned in the path of movement of the punch adjacent the end of its stroke and has a surrounding stripper element for removing the finished shell from the domer assembly after the doming operation has been completed. One of the problems of having a fixed domer element for reforming the end wall of a container shell is that difficulties may be encountered when a cup is initially misfed onto the punch which can cause serious damage to the domer assembly as well as the remaining elements of the press or machine.
In an effort to reduce the cost of finished containers, manufacturers have constantly been striving to reduce the thickness of the initial stock material thereby decreasing the metal cost of each container. Since the end wall (bottom) of the container essentially represents the initial thickness of the can stock, new (improved) bottom profiles are required to maintain the pressure performance of thinner gauge. The container must be capable of maintaining internal pressures approaching 100 PSI minimum without any significant distortion and, to provide such capability, rather elaborate dome or end configurations have been developed.
One of the more recent proposals for the end wall configuration of the container consists of forming an inclined portion adjacent the lower periphery of the side wall and then inwardly doming the center portion of the end wall so that the lowest most edge of the container shell is located inwardly of the side wall.
In order to produce the more elaborate configurations of end walls for container shells, more elaborate equipment is necessary to prevent wrinkling of the metal during the reforming process. One example of a domer assembly for reforming an end wall to the configuration discussed above, is disclosed in U.S. Pat. No. 3,771,345. This patent discloses a center domer element surrounded by an annular domer element and both elements are maintained in a predetermined position through a pair of piston and cylinder arrangements wherein air is introduced to apply the desired forces to the container shell in cooperation with the punch. Another example of a domer assembly is disclosed in U.S. Pat. No. 3,730,383 wherein a fixed domer element is surrounded by a movable annular element that is biased through an air cylinder.
With arrangements of this type, it is virtually impossible to determine whether there is any leakage across the seals that are necessary between the stationary cylinder and the movable piston. In order to determine whether the seals have been damaged, it is necessary to completely disassemble components for a visual inspection. Such a process is not only time consuming but is costly in terms of loss of production.
Another type of biasing mechanism that has been utilized for a center domer element and an annular domer element is a spring assembly of the type disclosed in U.S. Pat. No. 3,967,482. Again, it is extremely difficult with spring arrangements of this type to insure that there is an equal force being applied around the perimeter of the container shell to act as a hold down pressure during reformation of the center of the container shell end wall. If the force applied to the periphery of the container end wall during the reforming of the center wall is not uniform and equal for each container, wrinkles may be encountered which makes it necessary to discard the container. Wrinkles result primarily from the resistance of the metal to flow from the side wall of the container into the end wall around the annular peripheral portion which has a reducing diameter. Of course, it is difficult with a spring biasing arrangement to accurately determine whether the forces for the respective springs are equal.